Year 7 Science · Chemical Sciences · 9m read

Pure Substances and Mixtures

Ever wonder why salt dissolves in water but sand doesn't? That's because they're different types of matter. A pure substance has one ingredient (like pure water), while a mixture combines two or more (like saltwater). Understanding this distinction and learning separation techniques — filtering, distilling, chromatography — is essential for VCE chemistry and shows up everywhere from pharmaceutical development to water treatment plants.

Learning objective: Distinguish between pure substances and mixtures and describe separation techniques.

By the end of this lesson you can:

• I distinguish elements, compounds and mixtures.
• I classify mixtures as solutions, suspensions or colloids.
• I explain filtration, evaporation, distillation and chromatography.

Mixtures are salads; pure substances are single ingredients. In a salad you can pick out the tomato. In pure water there's nothing to pick out — it's all the same stuff. Separation techniques are ways of picking the salad apart.

Classification of matter

MATTER classification:

PURE SUBSTANCES — only ONE type of particle.
ELEMENTS: made of one type of atom. Cannot be broken down by chemical means.
Examples: gold (Au), oxygen (O₂), iron (Fe).
COMPOUNDS: two or more elements CHEMICALLY combined in fixed ratios.
Properties differ from their elements.
Water (H₂O): hydrogen gas + oxygen gas → liquid water.
Table salt (NaCl): sodium (reactive metal) + chlorine (toxic gas) → safe salt.

MIXTURES — two or more substances PHYSICALLY combined.
Substances RETAIN their properties. Can be separated physically.
Types of mixtures:
SOLUTION: solute dissolved in solvent. Homogeneous (uniform throughout).
Examples: salt water, orange juice, air.
SUSPENSION: undissolved particles in liquid. Settles over time.
Examples: muddy water, orange juice with pulp.
COLLOID: particle size between solution and suspension. Doesn't settle.
Examples: milk, fog, whipped cream, mayonnaise.

SEPARATION TECHNIQUES:
FILTRATION: separates insoluble solid from liquid (e.g. sand from water).
EVAPORATION: removes liquid solvent; solid remains (e.g. salt from seawater).
DISTILLATION: separates liquids with different boiling points (e.g. water from alcohol).
CHROMATOGRAPHY: separates mixtures by solubility differences (e.g. inks in markers).
MAGNETIC SEPARATION: using a magnet to remove magnetic materials (e.g. iron filings).

Separation technique guide

• Solid in liquid (insoluble): Use FILTRATION. The solid stays in the filter paper; the liquid passes through.
• Dissolved solid in liquid: Use EVAPORATION. Heat removes water; solid remains.
• Two liquids with different boiling points: Use DISTILLATION. Heat to the lower boiling point → one liquid evaporates first → collect it.

Quick reference

| Technique | Separates | Example |
|---|---|---|
| Filtration | Insoluble solid from liquid | Sand from water |
| Evaporation | Dissolved solid from liquid | Salt from seawater |
| Distillation | Liquid from solution | Pure water from salt water |
| Chromatography | Dissolved dyes | Ink colours |
| Magnetism | Magnetic materials | Iron from sulfur |
| Decanting | Liquid off sediment | Water off sand |

Worked example (Medium)

You have a mixture of salt and sand in water. Describe steps to obtain pure salt.

1. Step 1: Filtration. Filter the mixture through filter paper to remove sand (insoluble). The salt water passes through.
2. Step 2: Evaporation. Heat the salt water solution gently to evaporate the water. Pure salt crystals remain.

Answer: Filtration (removes sand) then evaporation (removes water) → pure salt.

Worked example (Hard)

A mixture contains iron filings, salt and sand. Describe a step-by-step method to separate all three components into pure substances.

1. Step 1 — Magnetic separation. Pass a magnet through the mixture to remove the iron filings. Iron is magnetic; salt and sand are not. Property exploited: magnetism.
2. Step 2 — Dissolving. Add water to the remaining salt and sand mixture and stir. Salt dissolves (it is soluble); sand does not (it is insoluble).
3. Step 3 — Filtration. Filter the mixture through filter paper. Sand remains on the filter paper (residue). Salt water passes through (filtrate). Property exploited: solubility.
4. Step 4 — Evaporation. Heat the salt water gently until the water evaporates. Pure salt crystals remain in the evaporating dish. Property exploited: different boiling points.

Answer: Magnetic separation (iron) → dissolving + filtration (sand) → evaporation (salt). Three techniques, each exploiting a different physical property.

Common mistakes

• Confusing elements and compounds Students think oxygen gas (O₂) is a compound because it has two atoms, but it is an element — both atoms are oxygen. Fix: An element has only ONE type of atom. O₂ is two oxygen atoms bonded → still an element. A compound has TWO OR MORE DIFFERENT types of atoms bonded (e.g. H₂O = hydrogen + oxygen).
• Saying mixtures are "chemically combined" Students confuse physical mixing with chemical bonding. Fix: Mixtures are PHYSICALLY combined — substances retain their own properties and can be separated by physical methods. Compounds are CHEMICALLY combined — a new substance with new properties is formed.
• Using the wrong separation technique Students apply filtration to dissolved substances or evaporation to insoluble solids. Fix: Match the technique to the PROPERTY: insoluble solid in liquid → filtration. Dissolved solid in liquid → evaporation. Two liquids with different boiling points → distillation.

Separation science in everyday life

Separation techniques are not just laboratory procedures — they are used in industries that affect your daily life in ways you might not realise.

Water treatment plants use multiple separation techniques in sequence. First, large debris is removed by screening (a form of filtration). Then, chemicals cause small particles to clump together (flocculation) so they can be filtered out. Finally, chlorination or UV treatment kills bacteria. Every glass of tap water you drink has been through this process.

Coffee making is filtration. Hot water dissolves flavour compounds from ground coffee beans. The filter separates the liquid coffee from the grounds. Espresso machines use pressure to force water through very finely ground coffee — a high-pressure filtration.

Crude oil refining uses fractional distillation on an industrial scale. Crude oil is a mixture of hundreds of hydrocarbons with different boiling points. In a distillation column, lighter fractions (petrol, LPG) rise to the top, while heavier ones (bitumen, diesel) remain lower. Every plastic product, fuel and pharmaceutical relies on this separation process.

Understanding separation science is understanding how the modern world processes raw materials into the products we depend on every day.

Practice questions

1. Which of the following is a COMPOUND?

   1. Oxygen (O₂)
   2. Gold (Au)
   3. Water (H₂O)
   4. Air

   (1 marks)

2. Describe the separation technique you would use to separate blue and black ink from a mixture, and explain why.

   (2 marks)

Summary

• Distinguish elements, compounds and mixtures.
• Classify mixtures as solutions, suspensions or colloids.
• Explain filtration, evaporation, distillation and chromatography.